The purpose of a voltage regulator is to provide a constant DC output voltage independent of the input voltage. One approach to this is to chop the input voltage in such a way that as the input voltage goes up, the duty cycle of the chopper is reduced so that the product of the two is kept constant. This chopped voltage is then filtered, producing a DC output voltage. The output voltage is then equal to the product of the input voltage and the duty cycle. This is called a constant volt-second regulator or an E.DELTA.t regulator. A simple block diagram of a volt-second regulator is shown in FIG. 1.
A description of a constant volt-second regulator appears below. The basic circuit described is shown in FIGS. 2 and 3. As the timing capacitor, C.sub.T, charges past some reference voltage, V.sub.C, the comparator driving the power switch turns off. The time required to charge C.sub.T to V.sub.C where V.sub.S is the unregulated supply voltage is ##EQU1## The cycle time is T=0.8 CV.sub.S R.sub.T /V.sub.S =K.sub.2. Thus, the duty cycle=K.sub.3 /V.sub.S and the product of duty cycle and input voltage is K.sub.3. The circuit as shown does not exactly obey this relationship because of the voltage offset of the diode. This is done as a simple means of avoiding the requirement that the comparators work with a zero input.
Note that: (1) The duty cycle timing and the full cycle timing are both performed by the same capacitor C.sub.T ; (2) The sense level, V.sub.C, is constant; and (3) The peak capacitor voltage is proportional to V.sub.S.
The advantage of this prior circuit is that it provides a semi-regulated output (.+-.several percent) over a wide range of input voltage without the need for a high gain feedback loop and all its associated complexity.
Unfortunately this circuit, and the ones using feedback, share the disadvantage of needing low power supplies to continuously drive the regulator circuit and the power switch. These supplies must be in operation before the main regulator will function and become a major part of the overall cost and complexity of the entire regulator.